Disk Utility doesn't support burning anymore, maybe that also encompasses erasing. Have you tried it within Finder? There might be an option when you right-click on the CD in the Finder.
If not, you can use Terminal. Just enter the following command for quick-erase:
I tried Finder again and this time it offered an option to "erase rewritable disc..." after right-clicking onto the remote disk option within the Finder sidebar.